When fine particles of metals are irradiated with light, a resonance absorption phenomenon known as plasmon absorption occurs. The absorption wavelength of this absorption phenomenon differs depending on the nature and shape of the metal particles. For example, it is known that a gold colloid containing fine spherical particles of gold dispersed in water has an absorption band near 530 nm, whereas if the shape of the fine particles is changed to a rod shape with a minor axis of 10 nm, then in addition to an absorption near 530 nm caused by the minor axis of the rods, the particles also exhibit an absorption at a longer wavelength caused by the major axis of the rods (S. S. Chang et al., Langmuir, 1999, 15, pp 701 to 709).
Conventional methods of synthesizing metal nanorods include electrochemical methods (Y. Y. Yu, S. S. Chang, C. L. Lee, C. R. C. Wang, J. Phys. Chem. B. 101, 6661 (1997)), chemical methods (N. R. Jana, L. Gearheart, C. J. Murphy, J. Phys. Chem. B, 105, 4065 (2001)), and photochemical methods (F. Kim, J. H. Song, P. Yang, J. Am. Chem. Soc., 124, 14316 (2002)). Furthermore, a method of preparing spherically shaped noble metal fine particles as colorants for coating materials and resin compositions, by dissolving a noble metal compound in a solvent, adding a high molecular weight dispersant, and then conducting a reduction is already known (Japanese Unexamined Patent Application, First Publication No. H11-80647). Moreover, a method that aims to form metal wiring patterns by supporting plasmon-absorbing inorganic fine particles on a solid surface and then growing the particles to a diameter of less than 100 nm and an aspect ratio of at least 1, thus generating fine rods, is also known (Japanese Unexamined Patent Application, First Publication No. 2001-064794).
An electrochemical method is a method in which metal ions eluted from the anode are reduced at the cathode, and then grown into rod-shaped fine particles through the action of a surfactant. This method requires an electrolysis apparatus, expensive conduction electrodes such as gold plates, platinum plates or silver plates, and an ultrasound irradiation device, and because the quantity of particles that can be manufactured is limited by the size of the apparatus, this method is unsuited to mass production. Furthermore, the aspect ratio of the metal nanorods varies with deterioration of the ultrasound irradiation device and variation in the quantity of silver eluted, meaning reproducibility tends to be problematic.
A chemical method is a method in which fine metal seeds are first prepared as growth nuclei, and these metal seeds are then added to a separately prepared growth liquid to grow rod-shaped particles. In this method, the usable time frame for the seeds is quite short, being only several hours, and the metal fine particles must be grown via a complex operation that involves stepwise growth with two or three stages. Moreover, other problems also arise in that reproducibility is difficult to achieve, and the concentration of the manufactured metal nanorods is low.
A photochemical method is a method in which metal ions in a surfactant-containing solution are irradiated with ultraviolet light over an extended period to generate metal nanorods. This method requires expensive equipment such as an ultraviolet light exposure apparatus. Furthermore, because production of the metal nanorods is restricted to the region undergoing light irradiation, there is a limit to the quantity of nanorods manufactured, meaning the method is unsuited to mass production.
In the production method described in Japanese Unexamined Patent Application, First Publication No. H11-80647, examples are presented in which amines are used as the reducing agent, but these reducing agents are not combined with a surfactant. Furthermore, although a high molecular weight dispersant is added, this dispersant is used as a protective colloid for the generated noble metal fine particles, and has no function in controlling the growth in the axial direction during formation of the noble metal fine particles. Accordingly, the noble metal fine particles that are manufactured are spherical fine particles, and rod-shaped metal fine particles are unobtainable.
The plasmon coloring of spherical gold fine particles manufactured using this method is blue, bluish-violet, or reddish violet, whereas spherical silver fine particles are yellow. These light absorption bands are limited to absorption in the vicinity of 530 nm for gold and absorption in the vicinity of 400 nm for silver, meaning there is a limit on the colors that can be used.
In addition, in conventional production methods, in those methods where metal fine particles are grown on the surface of a solid, the metal fine particles are supported on the solid surface, meaning they cannot be dispersed in various solvents or binders, and cannot be converted to paint form.